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## Copyright (C) 1996 John W. Eaton
##
## This file is part of Octave.
##
## Octave is free software; you can redistribute it and/or modify it
## under the terms of the GNU General Public License as published by
## the Free Software Foundation; either version 2, or (at your option)
## any later version.
##
## Octave is distributed in the hope that it will be useful, but
## WITHOUT ANY WARRANTY; without even the implied warranty of
## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
## General Public License for more details.
##
## You should have received a copy of the GNU General Public License
## along with Octave; see the file COPYING. If not, write to the Free
## Software Foundation, 59 Temple Place - Suite 330, Boston, MA
## 02111-1307, USA.
## usage: hankel (c, r)
##
## Return the Hankel matrix constructed given the first column
## c, and (optionally) the last row r.
##
## If the second argument is omitted, zeros are inserted below the main
## anti-diagonal. If the last element of c is not the same as the first
## element of r, the last element of c is used.
##
## See also: vander, sylvester_matrix, hilb, invhilb, toeplitz
## Author: jwe
function retval = hankel (c, r)
if (nargin == 1)
r = zeros (size (c));
elseif (nargin != 2)
usage ("hankel (c, r)");
endif
[c_nr, c_nc] = size (c);
[r_nr, r_nc] = size (r);
if ((c_nr != 1 && c_nc != 1) || (r_nr != 1 && r_nc != 1))
error ("hankel: expecting vector arguments");
endif
if (nargin == 1)
r (1) = c (length (c));
endif
if (c_nc != 1)
c = c.';
endif
if (r_nr != 1)
r = r.';
endif
nc = length (r);
nr = length (c);
if (r (1) != c (nr))
warning ("hankel: column wins anti-diagonal conflict");
endif
## This should probably be done with the colon operator...
retval = zeros (nr, nc);
for i = 1:min (nr, nc)
retval (1:nr-i+1, i) = c (i:nr);
endfor
tmp = 1;
if (nc <= nr)
tmp = nr - nc + 2;
endif
for i = nr:-1:tmp
retval (i, 2+nr-i:nc) = r (2:nc-nr+i);
endfor
endfunction
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